More on Gibbons

My previous posts, “Monkeying Around in Vietnam” and “Birding in Malaysia Turns Up Siamangs” mentioned several gibbon species that I saw there. These “lesser apes” are in the same superfamily as Humans, the Hominoidea, and branched off from our family line about 16 million years ago (MYA). There are 12-15 species (depending on the authority) in four genera. Compared with other apes, gibbons are small, slender, and agile, exhibit no sexual dimorphism, and have very long arms adapted for a spectacular arm swinging locomotion called “brachiation.” Gibbons live in small monogamous families composed of a mated pair and offspring and sometimes others that are assumed to be uncles and aunts, or possibly grandparents. Each family defends its territory from other families, and families in adjacent territories may meet at the boundary and yell at each other. This was the commotion that I heard on Fraser's Hill in Malaysia.
In all species, the mated pairs sing duets in which each gender's song is distinct from the other (Geissmann 2000, 2002). Each species sings a different, distinct duet. A pair's duet elicits duet calls from any other families that are within hearing.
Like humans, their monogamy is “dynamic,” with extra-pair copulations a regular, if infrequent, occurrence in all studied species (Palombit 1994a; Palombit 1994b; Rechard 1995; Sommer and Reichard 2000). Also like us, the males of mated pairs care for infants, at least in siamangs and hoolocks (Lappan 2008), and this may be a trait of the whole family. I have often wondered why human behavioural researchers and commentators draw inferences about our own behaviour from that of baboons (not even hominoids) and chimpanzees instead of gibbons that at least have a similar mating system.
For brevity, references are omitted here; please let me know if you would like them.